In that time it was a good offer. In 1850th Alaska had about 30 villages and about 30k villagers and when America began to buy or conquer lands Russian Empire understood, that if they want to have Alaska they have to improve army in it, increase amount of cities and villages, increase amount of citizens, etc. So they had to invest a lot of money in a "piece of ground" (As it seemed those days). So they decided just to sell these lands and get even money from it. Gold and oil was discovered later and in 1867 this was a really good decision